Compensating network for carrier transmission circuits



Jan. 11 1927. 1,613,607

H FLETCHER COMPENSATING NETWORK FOR CARRIER TRANSMISSION omcuns Filed Sept. 29, 1924 Harvey Heft/7e!" by mmyy Patented Jan. 11, 1927.

HARVEY FLETCHER, OF NEW YORK, N. Y

., ASSIGNGR T0 WESTERN ELECTRIC 001V"- PANY, INCORPORATED, OF NEW YORK, N. Y., A CGRPOBATION OF NEW YORK.

COMPENSATING NETWORK FOR CARRIER TRANSMISSION CIRCUITS.

Application filed September 29, 1924. Serial No. 740,533.

This invention relates to improvements in radio and wire broadcasting systems and. more particularly to means and methods for eliminating distortion occurring in such systems.

It is the object of this invention to cornpensate the distortion in the sound reproduction at the receiving station by caus ng an opposite kind of distortion in the trans mitted waves.

In practically all receiving sets in a radio broadcasting system there is a strong resonant response of the sound reproducen to detected currents in the frequency region near 1,000 cycles, the response being much less at higher frequencies and at lower frequencies. As a consequence, thequahty of both the music and speech transmitted is impaired. The sound reproducers are usually of the watch case or loud speaker type having a metallic diaphragm. The distortion 1n speech caused by the natural mechanical resonance of these diaphragms hasbeen found to be markedly similar for most receivers in common use, the mutual variations be ng quite negligible. Ordinarily compensat on for this distortion would be made in each individual receiver with resulting complication and expense. However, the similarity in distortion permits substantial improvements in compensation which can be realized by the use of a single device at the transmitting station, the characteristic of such device corresponding rather to the average of many receivers than to any single receiver.

It is an object of this invention, therefore, to provide an'arrangement at the broadcast ing station such that the distortion of all the receiving sets may be compensated at least in part.

The advantages of the invention will become more apparent when consideration is given to the accompanying drawing in which Fig. 1 shows a broadcasting circuit provided with a compensating arrangement in accordance with this invention and a number of associated receiving circuits and Fig. 2 shows some diagrams illustrative of the invention.

Referring to Fig. l a transmitting set is represented at 1, while 2, 3 and 4 represent a number of receiving circuits tuned for reception of the transmitted waves and located at various stations. At station 1 a series network consisting of inductance 5, capacity 6 and resistance 7 is bridged across the line between transmitter 8 and the input coil 9 of an amplifier 10. Transmitter 8 and coil 9 are of the type disclosed in application Serial No. 669,028 filed October 17,1923 by Charles R. Moore, but it is evident that any other desired combination of transmitter and input coil may be employed for the purposes of this invention. Modulator 11 for modulating a carrier wave supplied by a source 19, is located between the amplifier 1 0 and the antenna 12. The amplifier and modulartor circuits may be of any well known design, no particular arrangement being essential to the proper working of this invention. The waves radiated from antenna 12 are received at stations 2, 3 and 4 in the conven- I tional way and after detection and a1nplification if desired are reproduced as sound waves by receivers 16, 17 and 18, respectively.

Network 5, 6, 7 is so designed that it will add to the transmitter a loss characteristic ins :ead of a gain characteristic which approximates the gain curves of a numberof average receivers. This is illustrated in Fig. 2 wherein are shown the curves 13 and 14 as response or gain curves for two typical receivers. Curve 15 represents the loss pro-- duced by the network 5, 6, 7 Curves 13 and 14 are plotted with gain or response in napiers as ordinates against frequency as abscissae. For a definition of the napier as a unit of transmission and an explanation of its use, attention is directed to an article entitled The nature of speech and its interpre tation by Harvey Fletcher, published in vol.

193, #6, on page 729 of the Journal of the Franklin Institute, June, 1922.

In order more easily to compare the similarity of the characteristics of the loss curve of the network and the gain curves of the average receivers, curve 15, has been plotted in the same sense as curves 13 and 14.

Examination of the curves in Fig. 2 shows that in the region of strong response, 1,000 cycles, the response in napiers for all three curves is approximately the same and over the rest of the frequency range the mutual deviations are small. In other words there has been provided at the transmitting station a network having a response characteristic which substantially compensates the response characteristics of receivers employed at receiving stations. Actual measurements have shown that it is reasonable to expect that most of the receivers which are now being used have response curves similar to those of the typical receivers here illustrated. For practical purposes it has been found that a series of network consisting of an inductance of .007 henry, a condenser of capacity 3 mi. and a resistance of 65 ohms, fulfills the requirements of this invention. Curve 15 of Fig. 2 was plotted from calculations based on these values, and also upon the assumption that the impedance looking in either direction from the bridged network was 200 ohms. It is to be understood, however, that other suitable values may be used without departing from the spirit of the invention.

lVith a broadcasting system having this network, a decided improvement in quality will result in receiving sets whose sound reproducers have resonant peaks between frequency ranges of 900 and 1200. This would include a great majority of all the sets now in use. It may be thought that, while correcting the distortion occurring in a majority of receivers, the introduction of such a compensating device would cause a noticeable deterioration in the quality of sound in receiving sets which are practically perfect in response, that is, which have a flat characteristic. However, experiments indicate that the resulting effect will be almost negligible.

While this invention has been described with respect to a radio broadcasting system it is understood that it is equally applicable to systems for broadcasting over metallic wires or through the earth.

IVhat'is claimed is:

1. The method of compensating for the distortion of the acoustic response characteristics of a plurality of receiving circuits adapted to receive from the same broadcast transmitting circuit which comprises introducing at the transmitting circuit a complementary distortion corresponding to the normal distortion of the receiving circuits.

2. In a broadcasting system in which a plurality of receiving circuits are adapted to receive waves transmitted from a central broadcasting station, means at said central station for compensating the normal distortion occurring in said receiving circuits due to the tone selective characteristics of the signal reproducers.

3. In a broadcasting system in which a plurality of receiving circuits are adapted to receive waves transmitted from a central broadcasting station, a network bridged across the voice circuit at said central station for compensating the normal distortion occurring in said receiving circuits due to the tone selective characteristics of the signal reproducers.

4. In a broadcasting system in which a plurality of receiving circuits are adapted to receive waves transmitted from a central broadcasting station, a series resonant network bridged across the voice circuit at said central station for compensating the normal distortion occurring in said receiving circuits due to the tone selective characteristics of the signal reproducers.

5. In a radio broadcasting system in which a plurality of receiving circuits are adapted to receive waves transmitted from a central broadcasting station, means at said central station for compensating the normal distortion of said receiving circuits due to the tone selective characteristics of the signal reproducers, said means comprising a resistance, a capacity and an inductance in series.

6. A radio broadcasting transmitter having an antenna, a modulator, a source of high frequency waves and a source or" modulating current, a plurality of receiving circuits for the reception of waves transmitted trom' said broadcasting transmitter and means for compensating for the normal distortion due to the tone selective characteristics of the signal reproducers in said"receiving circuits comprising a series network consisting of resistance, capacity, and inductance bridged across said modulating current source.

7. In a broadcasting system in which a central station transmits intelligence to a plurality of receiving stations provided with receivers whose acoustic response characteristics are substantially similar, a network at said central station having a characteristic coinciding over practical limits with the response characteristics of said receivers but varying in the opposite sense for correcting distortion occurring in .said receivers.

8. The method of compensating at least in part the distortion in sound reproduction occurring in a plurality of reproducers arranged to receive waves from a central broadcasting transmitter which comprises distorting the transmitted waves in a sense opposite to that caused by the reproducers, and a magnitude equal to the normal distortion of the sound reproducers.

9. In a broadcasting system in which a central station transmits intelligence to a plurality of receiving stations equipped with telephone receivers which cause the reproduced sound to be distorted, means at said central station for distorting the transmitted waves in an opposite manner to that cause by said telephone receivers, and by an amount equal to the normal distortion of said receivers.

10. In a broadcasting system in which a central station transmits intelligence to a plurality of receiving stations equipped With telephone receivers which cause distortion in the reproduction of sounds, means at said central station consisting of a resonant network which causes in the transmitted Wave a distortion opposite to that produced by said telephone receivers, and by an amount equal to the normal acoustical distortion of said receivers.

In witness whereof, I hereunto subscribe my name this 23 day of September A. D.,

HARVEY FLETCHER. 

